For two years running, I found Continental to have the cheapest fares for me, leaving from Seattle. But that doesn’t mean it will work that way in your case. Read some posts here about using ITA Software, and try a one-month search, specifying different destinations. Unless you get a really good price, I probably wouldn’t buy now.

I ran ITA and found $779 total on Northwest/KLM to Dusseldorf and back from Cologne (back from DUS is $10 more). Compare with a good consolidator, such as Seattle-based www.travelteam.com but I doubt you’ll do better than $779 for this, or any, itinerary from Seattle outbound in early September. Of course if you wait too long to book the next-available fares will likely be more expensive.

In fact, ITA shows slightly better flight times if you go with KLM for $7 more ($786 rt total) via Amsterdam both ways, instead of via Detroit outbound like the $779 itinerary above. Northwest is starting new service to Dusseldorf from Detroit, but they’re using a smaller plane (B757) on the transatlantic portion, whereas the KLM flight between Seattle and Amsterdam (nonstop) is actually operated by partner Northwest using Airbus 330s which are more comfortable (IMO) for transatlantic flights. Even though NW is setting up their new 757s with above-average kneeroom (34-35” pitch, vs. 31-32” most other airlines), I’d still go for their nonstop SEA-AMS flights on the transatlantic portions.